The present invention relates to casting a near-net-shape article. More particularly, the present invention relates to an apparatus for casting a near-net shape article, such as, for example, an airfoil for a gas turbine assembly, from high-temperature materials, such as niobium-base suicides and molybdenum-base silicides. The present invention also relates to methods for casting a near-net-shape article.
In a gas turbine assembly, such as an aeronautical turbine, a land-based turbine, a marine-based turbine, and the like, compressed air is mixed with fuel in a combustor and ignited, generating a flow of hot combustion gases through one or more turbine stages that extract energy from the gas, producing output power. Each turbine stage includes a stator nozzle having vanes that direct the combustion gases against a corresponding row of turbine blades extending radially outwardly from a supporting rotor disk. The vanes and blades include airfoils having a generally concave xe2x80x9cpressurexe2x80x9d side and a generally convex xe2x80x9csuctionxe2x80x9d side, both sides extending axially between leading and trailing edges over which the combustion gases flow during operation. The vanes and blades are subject to substantial heat load, and, because the efficiency of a gas turbine assembly is a function of gas temperature, the continuous demand for efficiency translates to a demand for airfoils that are capable of withstanding higher temperatures for longer service times.
Many components of gas turbine assemblies, including turbine airfoils on such components as vanes and blades, are usually made of superalloys. The term xe2x80x9csuperalloyxe2x80x9d is usually intended to embrace iron-, cobalt-, or nickel-based alloys, which include one or more other elements, including such non-limiting examples such as aluminum, tungsten, molybdenum, titanium, and iron. Superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. However, turbine components such as, for example, airfoils in modern jet enginesxe2x80x94can only operate at temperatures as up to about 1,150xc2x0 C., which is about 85% of the melting temperatures of most Ni-based superalloys. Furthermore, advanced design concepts call for turbine component operating temperatures beyond the capabilities of state-of-the-art superalloys to maintain properties such as strength and creep resistance in a required range. Therefore, there is a desire in the industry to exploit the advantages of certain new classes of materials for use in gas turbine assemblies.
Refractory metal intermetallic composite materials, including, but not limited to, those comprising silicon (Si) and at least one of niobium (Nb) and molybdenum (Mo), possess a useful range of mechanical properties in the temperature range required by turbine assembly designs. Examples of materials of this type include, for example, those described in U.S. Pat. Nos. 5,833,773 and 5,932,033. Although the properties of this class of materials are attractive for use in advanced high-temperature applications, such as gas turbine assemblies, processing such materials, especially to near-net shapes, remains a significant technical challenge. Therefore, what is needed is an apparatus and a method to produce articles, such as components of turbine assemblies, in near-net-shape form. What is also needed is an apparatus and a method to form refractory metal intermetallic composite materials into near-net shape articles.
The present invention meets this and other needs by providing a method and apparatus for casting a near-net-shape article from a high temperature material, such as a refractory metal intermetallic composite material. The near-net shape article may be a turbine assembly component, such as, but not limited to, a vane or an airfoil.
Accordingly, one aspect of the invention is to provide an apparatus for casting an article. The apparatus comprises: a means for forming a molten material, the molten material comprising at least one of a metal and an alloy; a means for pouring the molten material from the means for forming the molten material, wherein the means for pouring the molten material includes a cup for receiving the molten material in a superheated state; a mold assembly for receiving the molten material from the cup, the mold assembly comprising a solid shell for containing the molten material, the shell having a face coat, wherein the face coat is disposed on an inner surface that contacts the molten material such that the face coat is interposed between the solid shell and the molten material, the face coat comprising at least one of an oxide, a silicide, a silicate, an oxysulfide, and a sulfide and containing at least one of a rare earth metal, a refractory metal, and combinations thereof, and a heater assembly for maintaining the solid shell at a predetermined temperature, wherein the molten material solidifies within the cavity to form a near-net shape of the article.
A second aspect of the invention is to provide a mold assembly for casting an article from a molten material. The mold assembly comprises: a mold comprising a ceramic shell defining a cavity therein for receiving the molten material, and at least one face coat of a refractory material disposed on an inner surface of the ceramic shell in the cavity such that the face coat is interposed between the solid shell and the molten material, the refractory material comprising at least one of hafnia, erbia, zircon, yttria, ceria, zirconia, and at least one stabilized zirconia; and a heater assembly for heating the mold to a predetermined temperature and controlling a temperature of the mold, the heater assembly being proximate to an outer surface of the mold.
A third aspect of the invention is to provide an apparatus for casting an article. The apparatus comprises: a means for forming a molten material, the molten material comprising at least one of a metal and an alloy; a means for pouring the molten material from the means for forming the molten material, wherein the means for pouring the molten material includes a cup for receiving the molten material in a superheated state; a mold assembly for receiving the molten material from the cup, the mold assembly comprising a ceramic shell for containing the molten material, the ceramic shell defining a cavity therein for accepting the molten material, and at least one face coat of a refractory material disposed on an inner surface of the ceramic shell in the cavity such that the face coat is interposed between the solid shell and the molten material, the refractory material comprising at least one of hafnia, erbia, zircon, yttria, ceria, zirconia, and stabilized zirconia, wherein the face coat contacts the molten material; and a heater assembly for heating the mold to a predetermined temperature and controlling a temperature of the mold, the heater assembly being proximate to an outer surface of the mold, wherein the molten material solidifies within the cavity to form a near-net shape of the article.
A fourth aspect of the invention is to provide a method of casting a near-net shape article. The method comprises the steps of: forming a molten material comprising at least one of a metal and an alloy; pouring the molten material into a cup; maintaining the molten material in a superheated state within the cup; receiving the molten material from the cup in a mold assembly, wherein the mold assembly comprises a solid shell for containing the molten material, the solid shell having a face coat disposed on an inner surface of the solid shell that contacts the molten material, the face coat comprising at least one of an oxide, a silicide, a silicate, an oxysulfide, and a sulfide and containing at least one of a rare earth metal, a refractory metal, and combinations thereof; and solidifying the molten material in the mold assembly to form the near-net shape of the article.
A fifth aspect of the invention is to provide a method of casting a near-net shape article from at least one alloy comprising silicon and at least one of niobium, molybdenum, titanium, and chromium. The method comprises the steps of: providing the at least one alloy comprising silicon and at least one of niobium, molybdenum, titanium, and chromium; forming a molten material comprising the alloy; pouring the molten material into a cup; maintaining the molten material in a superheated state within the cup; receiving the molten material from the cup in a mold assembly, wherein the mold assembly comprises a ceramic shell having a cavity for receiving the molten material and at least one face coat of a refractory material disposed on an inner surface of the ceramic shell in the cavity, wherein the refractory material comprises at least one of hafnia, erbia, zircon, yttria, ceria, zirconia, and stabilized zirconia; and solidifying the molten material in the mold assembly to form the near-net shape article.
A sixth aspect of the invention is to provide a near-net shape article comprising at least one alloy comprising silicon and at least one of niobium, molybdenum, titanium, and chromium. The near-net shape article is cast by: forming a molten material comprising the at least one alloy; pouring the molten material into a cup; maintaining the molten material in a superheated state within the cup; receiving the molten material from the cup in a mold assembly, wherein the mold assembly comprises a ceramic shell and at least one face coat of a refractory material disposed on an inner surface of the ceramic shell in the cavity, wherein the refractory material comprises at least one of hafnia, erbia, zircon, yttria, ceria, zirconia, and stabilized zirconia; heating the mold assembly to a temperature at which the molten material is in a fluid state; and solidifying the molten material in the mold assembly to form a near-net shape article.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.